We present Aerial, a tool for the online monitoring of metric regular properties. Aerial supports both the standard metric temporal logic (MTL) and the more expressive metric dynamic logic (MDL) as its property specification language. Unlike MTL, which is restricted to star-free properties, MDL can express all metric regular properties by generalizing MTL's temporal operators to arbitrary regular expressions. Aerial's distinguishing feature is its ability to monitor an event stream using memory logarithmic in the event rate. This memory efficiency is achieved by altering how Aerial outputs its monitoring verdicts.
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